Fingerprint sensing device with heterogeneous coating structure comprising a dielectric material

ABSTRACT

The invention relates to a fingerprint sensing device comprising a sensing chip comprising an array of capacitive sensing elements. The sensing device comprises a coating material arranged in a layer on top of the array of sensing elements and comprising a plurality of cavities filled with a dielectric material. The dielectric material comprises reduced graphene oxide. Locations of the cavities correspond to locations of the sensing elements such that a cross-section area of a cavity covers at least a portion of an area of a corresponding sensing element. A dielectric constant of the dielectric material is higher than a dielectric constant of the coating material. The invention also relates to a sensing device where the dielectric coating layer containing reduced graphene oxide comprises trenches corresponding to areas between the sensing pixels filled with a fill material, where the dielectric coating layer has a higher dielectric constant than the fill material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 15/138,525 filed Apr. 26, 2016, which claims the benefit ofSwedish Patent Application No. 1550749-4 filed Jun. 8, 2015. The entiredisclosures of the above applications are incorporated herein byreference.

FIELD

The present invention relates to a coating structure for a fingerprintsensor. In particular, the present invention related to a heterogeneouscoating structure for enhancing the performance in a fingerprint sensor.

BACKGROUND

As the development of biometric devices for identity verification, andin particular of fingerprint sensing devices, has led to devices whichare made smaller, cheaper and more energy efficient, the possibleapplications for such devices are increasing.

In particular fingerprint sensing has been adopted more and more in, forexample, consumer electronic devices, due to small form factor,relatively beneficial cost/performance factor and high user acceptance.

Capacitive fingerprint sensing devices, built based on CMOS technologyfor providing the fingerprint sensing elements and auxiliary logiccircuitry, are increasingly popular as such sensing devices can be madeboth small and energy efficient while being able to identify afingerprint with high accuracy. Thereby, capacitive fingerprint sensorsare advantageously used for consumer electronics, such as portablecomputers, tablet computers and mobile phones, e.g. smartphones.

A fingerprint sensing chip typically comprises an array of capacitivesensing elements providing a measure indicative of the capacitancebetween several sensing structures and a finger placed on the surface ofthe fingerprint sensor. The sensing chip may further comprise logiccircuitry for handling addressing of the array of sensing elements.

A typical fingerprint sensor is protected so that the fingers do notcome in physical contact with the sensing elements. In particular, itmay be desirable to arrange a glass plate on top of the sensor forprotecting the sensor, or to arrange the sensor behind a display glass.By arranging elements between the sensing surface and the sensingelements, the distance between the sensing surface and the sensingelements increases which reduces the capacitive coupling between afinger placed a sensing surface of the device and the capacitive sensingelements. This in turn leads to an image blurring effect. As a functionof an increased distance between a finger and any given pixel, eachpixel is starting to receive signals from areas that are not immediatelylocated vertically on top of said pixel resulting in image blurringnegatively impacting the capabilities of the sensors to resolve finefeatures in a fingerprint.

In view of the above, it is desirable to improve the capacitive couplingbetween a finger placed on the sensing surface and the sensing elements.

US2013/0201153 discloses a fingerprint sensing device where electricallyconductive strands are arranged between the sensing surface and thesensing elements of a fingerprint sensing device. An insulating materialis arranged between conductive strands. However, a direct electricalcontact between the finger and the pixel may cause problems related toelectrostatic discharge (ESD). Moreover, the metallic portions of thesurface may oxidize, resulting in undesirable aesthetic effects.

SUMMARY

In view of above-mentioned desirable properties of a fingerprint sensingdevice, and drawbacks of prior art, it is an object of the presentinvention to provide a fingerprint sensing device and a method formanufacturing a fingerprint sensing device which provides an improvedcapacitive coupling between a finger placed on a sensing surface and thesensing elements of the sensing device.

According to a first aspect of the invention, there is provided afingerprint sensing device comprising: a sensing chip comprising anarray of sensing elements, the sensing elements being configured to beconnected to readout circuitry for detecting a capacitive couplingbetween each of the sensing elements and a finger placed on a sensingsurface of the sensing device; a coating material arranged in a layer ontop of the array of sensing elements, the coating material comprising aplurality of cavities filled with a mold material, wherein the moldmaterial is deposited by compression molding; wherein locations of thecavities correspond to locations of the sensing elements, such that across-section area of a cavity covers at least a portion of an area of acorresponding sensing element; and wherein a dielectric constant of themold material is higher than a dielectric constant of the coatingmaterial.

There is also described a method for manufacturing a fingerprint sensingdevice, the method comprising; providing a sensing chip comprising anarray of sensing elements, the sensing elements being configured to beconnected to readout circuitry for detecting a capacitive couplingbetween each of the sensing elements and a finger placed on a sensingsurface of the sensing device; depositing a layer of a coating materialcovering the array of conductive sensing elements; forming a pluralityof cavities in the coating material, wherein locations of the cavitiescorrespond to locations of the sensing elements such that across-section area of a cavity covers at least a portion of an area of acorresponding sensing element; and filling the cavities with adielectric material, the dielectric material having a dielectricconstant higher than a dielectric constant of the coating material.

The dielectric material may be a mold material or an adhesive material.In the following, the dielectric material will mainly be referred to asa dielectric mold material comprising reduced graphene oxide. However,the skilled person realizes that the described embodiments may beequally applicable for an adhesive material comprising reduced grapheneoxide.

The dielectric material matrix may be a polymer that can be patterned byphoto lithography. Example materials include polyimide, PMMA, PVDF. Thematerial may be a photoresist so that once it is spin-cast onto thesensing array comprising the pixels (such as a SiN surface) it can beexposed in the same pattern as the pixel mask. This will allow thedielectric material to only cover the areas over the pixels and therebyimprove the signal focusing through the overall package stack.

The concentration of reduced graphene oxide (rGO) would depend on thedesired dielectric constant and dielectric loss values. Depending on thespecific sensor stack-up (height and other materials over the sensor)the dielectric constant and dielectric loss values desired could betuned specifically to fit a specific stack-up. Approximately 0.01-10 wt% rGO would be added to increase the dielectric constant in the range of10 to 100. The dielectric properties can also be tuned by thermallyreducing the graphene oxide more or less. An additional advantage isthat partially reduced GO can be added to the polymer matrix and thenin-situ reduced when it has been deposited on the wafer level and thenit can be reduced more during the polymer curing process. Additionalfillers can also be added, such as barium titanate, in order to furtherimprove the dielectric constant of the entire composite.

The sensing chip should in the present context be understood as a chipcomprising a plurality of sensing elements in the form of conductiveplates or pads, typically arranged in an array, which are capable offorming a capacitive coupling between each sensing element and a fingerplaced on an exterior surface of the fingerprint sensing device. Throughreadout of the capacitive coupling for each sensing element, ridges andvalleys of a fingerprint can be detected as a result of the distancedependence of the capacitive coupling. To achieve a fingerprint imagewith sufficient resolution, the sensing elements are typicallysubstantially smaller than the features (ridges and valleys) of thefinger. In general, a chip may also be referred to as a die.

The sensing device according to various embodiments of the invention maybe formed on a conventional rigid PCB substrate or it may be implementedusing a flexible type of substrate.

An improved capacitive coupling between a finger and a sensing elementcan be achieved by forming a heterogeneous coating layer where portionsof the layer above the sensing elements have a higher dielectricconstant than surrounding portion, thereby focusing the electric fieldtowards the respective sensing element. Furthermore, the presentinvention is based on the realization that the mold used to cover andprotect the sensing device can be used to achieve this effect byselecting or forming a mold material having a dielectric constant whichis higher than the surrounding coating material. Thereby, an improvedcapacitive coupling can be achieved without substantial alterations ofthe material stack, meaning that conventional manufacturing processesmay be used.

That a cross-section area of a cavity covers at least a portion of anarea of a corresponding sensing element means should be interpreted tomean that the cavity may or may not cover the complete area of thesensing element. Moreover, it is not required that the cavity iscentered over the sensing element, although it very well may be.

Furthermore, it is important to note that the cavity should beunderstood as a cavity in the coating material, which is subsequentlyfilled with a mold material.

The coating material may refer to any material which is arranged tocover the sensing chip and in particular the sensing elements. Thecoating material is often referred to as wafer coating.

The coating material may comprise one cavity for each sensing element.Although it is not strictly required that there is a 1:1 ratio of thenumber of cavities to the number of sensing elements, this is mostlikely how the greatest improvement in capacitive coupling can beachieved. However, there may be instances where it is desirable to onlyhave cavities over some of the sensing elements. For example, forvarious reasons it may be difficult to separate adjacent cavities, inwhich case a pattern where cavities are only located above a selectnumber of sensing elements can be utilized.

The step of filling the cavities with a mold material may advantageouslycomprise compression molding. Moreover, the step of compression moldingmay comprise dispensing granulated mold particles on the layer ofcoating and in the cavities; heating the mold granulate and applying apressure to the mold granulate. Compression molding enables filling ofsmall structures such as the cavities of the present sensing device. Incomparison, transfer molding, also referred to as injection molding,could most likely not be used to fill the cavities of the presentstructure since in transfer molding, the mold is required to travel adistance before reaching some of the cavities to fill. As the mold istransferred, it also begins to cure, which makes transfer moldingunsuitable for filling the type of cavities discussed herein.

The coating layer may advantageously be deposited by spin coating or byspray coating, which can be done on a full wafer thereby providing alarge-scale efficient process. Using spin coating or spray coating alsoallows the process to be easily modified with respect to the desiredthickness of the coating layer. The coating material is preferablyarranged in a homogeneous layer on the sensing chip to cover the sensingelements, which also can be achieved by spin and spray coating.

According to one embodiment of the invention, the dielectric constant ofthe dielectric material may be in the range of 5-100 and the dielectricconstant of the coating material may be in the range of 2-5. Thespecified ranges are should be seen as exemplary ranges providing thedesired effect. The mold material and the coating material may havedielectric constants outside of the specified ranges within the scope ofvarious embodiments of the present invention.

Furthermore, the ratio between the dielectric constant of the dielectricmaterial and the dielectric constant of the coating material mayadvantageously be selected to be equal to or larger than 2:1. Withrespect to the focusing effect, it is the ratio between the twodielectric constants which determines the amount of focusing, where ahigher ratio provides a better focus. It should be noted that the abovementioned dielectric constants and ratio is merely an example, and thata desired advantageous effect can be achieved with in principle anyratio higher than 1, although the effect is increasing with increasingratio.

In one embodiment of the invention, the dielectric material mayadvantageously comprise filler particles having a dielectric constanthigher than an average dielectric constant of the mold material, whichis one way of tailoring the average dielectric constant of the moldmaterial. The filler particles may be referred to as dielectric fillerparticles or high-k filler particles. Thereby, the dielectric constantof the mold material can be selected so that a desirable ratio can beachieved for different choices of coating material. Moreover, one andthe same dielectric material can be used while providing differentdielectric constants depending on what is required for a particularapplication. This simplifies the manufacturing process since there is noneed to adjust the process for different mold materials.

According to one embodiment of the invention, the filler particles mayadvantageously comprise reduced graphene oxide. One desirable propertyis that the filler material should be possible to be provided in a formwhich may be evenly mixed with a mold material, and that the fillermaterial does not agglomerate in the mold material since it is importantthat the dielectric constant of the mold material is at leastapproximately homogeneous over the entire surface of the sensing device.

In one embodiment of the invention, each of the cavities mayadvantageously comprise at least one lateral opening connecting thecavity to at least one adjacent cavity, enabling a flow of thedielectric material between adjacent cavities when depositing thedielectric material. During manufacturing of the fingerprint sensingdevice, a mold material is provided by compression molding where themold is heated to become fluid and then compressed to fill the cavitiesin the coating layer. It is desirable to achieve a homogeneous thicknessdistribution of the mold materiel to provide uniformity in measurementsover the entire sensing surface. In particular, it is desirable to avoidair-filled cavities. By means of the lateral openings in the coatinglayer, fluidly connecting adjacent cavities, the mold material can flowbetween the cavities to form an even mold layer covering the sensingdevice.

The openings are preferably larger than a maximum size of the fillerparticles in the mold material. Since it is desirable to achieve an evendistribution of the mold material, it is preferred that the openingsbetween adjacent cavities are larger than the dielectric fillerparticles so that the filler particles may flow freely between adjacentcavities, and that they not block the openings.

The coating material may advantageously be a photoresist. By using aphotoresist, the cavities can be formed using conventionalphotolithography and development processes, which simplifies the overallprocess flow. Moreover, a photoresist can easily be tailored to have aspecific dielectric constant so that a desired ratio of dielectricconstants can be achieved. Furthermore, a photoresist can be depositedon a full wafer with a high degree of accuracy and thickness uniformity,using for example spin coating or spray coating.

The method may further comprise plasma cleaning of the coating materialprior to the step of providing the mold material. The plasma cleaning ofthe surface of the coating material improves the wetting of the coatingwhich in turn improves the filling of the cavities by the mold material.Plasma cleaning also provides a surface with improved adhesion to themold material.

The method may further comprise depositing an adhesive on the moldmaterial; and attaching a protective plate to the fingerprint sensingdevice by means of the adhesive. The protective plate typicallycomprises a dielectric material in order to provide a good capacitivecoupling between a finger placed on the plate and the sensing elementsof the sensing chip. In particular, the protective plate mayadvantageously comprise a glass or ceramic material, such as achemically strengthened glass, ZrO₂ or sapphire. The aforementionedmaterials all provide advantageous properties in that they are hard andthereby resistant to wear and tear, and in that they are dielectricthereby providing a good capacitive coupling between a finger placed onthe surface of the protective plate and the sensing element of thesensing device. The protective plate described herein commonly forms theouter surface of the fingerprint sensing device, also referred to as thesensing surface.

Moreover, the adhesive used to attach the protective plate to thesensing chip may advantageously have the same dielectric constant as themold material, so that it does not influence the field propertiesbetween a protective plate and the sensing elements.

According to a second aspect of the invention, there is provided afingerprint sensing device comprising: a sensing chip comprising anarray of sensing elements, the sensing elements being configured to beconnected to readout circuitry for detecting a capacitive couplingbetween each of the sensing elements and a finger placed on a sensingsurface of the sensing device; a dielectric material arranged in a layeron top of the array of sensing elements, the dielectric materialcomprising reduced graphene oxide, the dielectric material furthercomprising a plurality of trenches filled with a fill material, whereinlocations of the trenches correspond to areas between adjacent sensingelements; and wherein a dielectric constant of the fill material islower than a dielectric constant of the coating material.

The fill material arranged in the trenches of the dielectric materialmay be an adhesive or a mold material. An adhesive may for example beused to attach a protective plate to the fingerprint sensing device.

Additional advantages, effects and features of the second aspect of theinvention are largely analogous to those described above in connectionwith the first aspect of the invention.

There is also described a method for manufacturing a fingerprint sensingdevice, the method comprising; providing a sensing chip comprising anarray of sensing elements, the sensing elements being configured to beconnected to readout circuitry for detecting a capacitive couplingbetween each of the sensing elements and a finger placed on a sensingsurface of the sensing device; depositing a layer of a dielectricmaterial covering the array of conductive sensing elements, thedielectric material comprising reduced graphene oxide; forming aplurality of trenches in the dielectric material, wherein locations ofthe trenches correspond to areas between adjacent sensing elements;filling the trenches with a fill material, the fill material having adielectric constant lower than a dielectric constant of the dielectricmaterial.

The fill material may be an adhesive film such as a die attach film(DAF), a liquid adhesive of a mold material.

The trenches in the dielectric material can be considered to follow thealignment of the border between sensing elements. Typically, the sensingelements are arranged in a square array with a certain pitch, heredefined as the center-to-center distance of the sensing elements, wherethe pitch is larger than the size of the sensing element, therebyforming an unoccupied area between adjacent sensing elements.

By providing a fill material in the trenches, where the dielectricconstant of the fill material is lower than a dielectric constant of thedielectric material, a heterogeneous coating layer is provided and thefocusing effect discussed in relation to the first aspect of theinvention is achieved.

Moreover, the step of filling the trenches with a mold material mayadvantageously comprise compression molding.

Further features of, and advantages with, the present invention willbecome apparent when studying the appended claims and the followingdescription. The skilled person realize that different features of thepresent invention may be combined to create embodiments other than thosedescribed in the following, without departing from the scope of thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present invention will now be describedin more detail, with reference to the appended drawings showing anexample embodiment of the invention, wherein:

FIG. 1 schematically illustrates a handheld electronic device comprisinga fingerprint sensing device according to an embodiment of theinvention;

FIGS. 2a-b schematically illustrate a fingerprint sensing deviceaccording to an embodiment of the invention;

FIGS. 3a-b schematically illustrate a fingerprint sensing deviceaccording to embodiments of the invention;

FIG. 4 is a flow chart outlining the general steps of a method formanufacturing a fingerprint sensing device according to an embodiment ofthe invention;

FIGS. 5a-d schematically illustrate a method for manufacturing afingerprint sensing device according to an embodiment of the invention;

FIG. 6 schematically illustrates a fingerprint sensing device accordingto an embodiment of the invention;

FIG. 7 is a flow chart outlining the general steps of a method formanufacturing a fingerprint sensing device according to an embodiment ofthe invention;

FIGS. 8a-c schematically illustrate a method for manufacturing afingerprint sensing device according to an embodiment of the invention;

FIG. 9 schematically illustrates a fingerprint sensing device accordingto an embodiment of the invention; and

FIGS. 10a-b schematically illustrate details of a fingerprint sensingdevice according to embodiments of the invention.

DETAILED DESCRIPTION

In the present detailed description, various embodiments of afingerprint sensing device according to the present invention are mainlydiscussed with reference to a capacitive fingerprint sensing device. Amethod for manufacturing a fingerprint sensing device is also discussed.

FIG. 1 is a schematic illustration of a handheld device 100 comprising afingerprint sensing device 102 comprising a touchscreen display 104. Afingerprint sensing device 102 can be used in for example a mobilephone, a tablet computer, a portable computer or any other electronicdevice requiring a way to identify and/or authenticate a user.

FIG. 2 is a schematic illustration of a fingerprint sensing device 200according to an embodiment of the invention. The exterior surface of thesensing device 200 is referred to as the sensing surface, since that isthe surface where a finger will be placed for capturing a fingerprintimage. The fingerprint sensing device is based on a sensing chip 202comprising an array of sensing elements 204. The sensing elements 204are here shown arranged in a square array, the sensing elements having asize of about 50×50 μm and a distance between adjacent elements is about5 μm. The sensing elements 204 are electrically conductive, typicallymetallic, and can as a general approximation be considered to act as oneplate in a parallel plate capacitor, where a finger placed on a sensingsurface of the fingerprint sensing device 200 represents the otherplate. Each sensing element 204 is connected to readout circuitry (notshown) for detecting a capacitive coupling between each of said sensingelements 204 and a finger placed on the sensing surface 201.

A coating material 205 is arranged in a layer on top of the array ofsensing elements 204, and the coating material comprises a plurality ofcavities 206 which are filled by a dielectric material 208 covering andprotecting the sensing elements 204, thereby forming the outer surfaceof the fingerprint sensor 200.

The cavity 206 may also be referred to as an opening, or a recess, inthe coating material 205. The purpose of the cavities 206 is to allow adielectric material to be arranged directly above the sensing elements204, but not in the areas in between sensing elements, so that thedielectric material 208 is arranged between the sensing element 204 andthe sensing surface 201 in the vertical direction. The dielectricmaterial 208, which has a dielectric constant that is higher than adielectric constant of the coating material 205, will then act as afocusing element helping to focus the electromagnetic field linesbetween a finger and the sensing element 204 towards the sensing element204. This effect is further illustrated in FIG. 2b showing a side viewof the fingerprint sensing device 200 where a ridges and valleys of afinger 212 are located on the sensing surface 201. It can be seen thatthe field lines 214 originating in a position on the sensing surface notlocated directly above a sensing element 204 are curved towards thecavities in the coating comprising the dielectric material 208 due tothe higher dielectric constant of the dielectric material 208. Moreover,the coating material 205 having a lower dielectric constant than thedielectric material 208 acts as a blocking structure in order to reduceor prevent field lines from a fingerprint ridge reaching a sensingelement 204 not located directly beneath the ridge. Accordingly, thepatterned coating layer 205 helps to reduce or prevent blurring of acaptured image, since the non-vertical coupling between the finger 212and the sensing elements 204 is reduced. In FIG. 2b , the field is lowerin the coating material 205 compared to in the dielectric material 208,due to the difference in dielectric constant.

In principle, it is the ratio between the dielectric constants of thecoating material 205 and the dielectric material 208 which determinesthe distribution of the field lines. Already a ratio of 2:1 provides anadvantageous effect, whereas a ratio in the range of 1:10 to 1:20 ismore preferable. The dielectric constants of the materials discussedherein are the average relative dielectric constants of the material.The respective materials may for example be compositions and compriseparticles having individually different dielectric constants, whichtogether with the bulk material provide a resulting average dielectricconstant. For example, a dielectric material with an increaseddielectric constant can be achieved by using a conventional dielectricmaterial and by adding particles comprising reduced graphene oxide. Theincreased dielectric constant can also be achieved by providing aferroelectric material such as barium titanate (BaTiO3) which in itselfhas a dielectric constant above 1000. By selecting the type andconcentration of the added dielectric particles, the dielectric materialand also the coating material can be tailored to have the desireddielectric constant within a reasonable range, such as between 2 and100. The resulting dielectric constant ε_(eff) for a mixture ofcomponents having different dielectric constants ε₁, ε₂, can bedetermined according to the Lichtenecker model aslog ε_(eff)=ν₁ log ε₁+ν₂ log ε₂where ν₁ and ν₂ are empirically determined constants.

FIG. 3a is a schematic illustration of a fingerprint sensing deviceaccording to an embodiment of the invention where adjacent cavities 206in the coating material are connected via channels 306, or openings 306,in the side walls of the cavities. The channels 306 allow a liquiddielectric material to flow between adjacent cavities during compressionmolding or during deposition of a fluid adhesive material, as will bediscussed in further detail in relation the method for manufacturing afingerprint sensing device. The openings 306 between adjacent cavitiesare configured to be larger than the particle size of any fillerparticles present in the dielectric material 208, so that the dielectricmaterial can flow freely between the cavities without the risk of fillerparticles clogging the openings. Preferably, the openings have a sizelarger than a maximum size of the filler particles in the dielectricmaterial. A typical maximum particle size may be in the range of 1-3 μmfor ferroelectric particles such as BaTiO₃ particles. However, fillerparticles having a high dielectric constant may also be provided in theform of nanoparticles having a sub-μm diameter. Accordingly, theopenings 306 between adjacent cavities can be selected based on the sizeof the filler particles and based on the method for patterning thecoating layer, and a practical size of the openings 306 may be in therange of 5-10 μm. Furthermore, the dielectric material 208 may compriseadditional filler particles in order to tailor parameters such as theviscosity and the thermal expansion coefficient of the dielectricmaterial. The openings may be adapted to have a size larger than amaximum size of also such filler particles. However, it is prioritizedto ensure that dielectric particles influencing the dielectric constantof the dielectric material can flow freely so that a homogeneousdielectric constant can be achieved in the dielectric material over thefull area of the sensing chip.

FIG. 3b is a schematic illustration of a fingerprint sensing deviceaccording to an embodiment of the invention where the openings 308connecting adjacent cavities 206 in the coating material are located atthe corners of the sensing elements 204, i.e. in the corners of thecavities 206. It should be understood that the openings connectingadjacent cavities may be configured in many different ways to achievethe desired effect of allowing the mold to flow between adjacentcavities.

FIG. 4 is a flow chart outlining the general steps of a manufacturingmethod according to an embodiment of the invention. The manufacturingmethod will be discussed also with reference to FIGS. 5a -d.

First, in step 402, a sensing chip 202 is provided and a coating layeris deposited 404 onto the sensing chip 202. The coating layer typicallyhas a uniform thickness and is arranged to cover the entire area of thesensing chip. The coating layer can for example be a photoresistdeposited by spin coating, and the photoresist may be either a positiveor a negative photoresist.

Cavities 206 are formed 406 in the coating layer 205 by means ofconventional photolithography and subsequent development to formcavities having the desired shape and distribution, as exemplified inFIG. 5a . Typically, the cavities are configured to reach through thecoating layer to expose the sensing element. Moreover, the sensingelement may be covered by a silicon nitride-based passivation layer (notshown) which is well known in the field of CMOS-processing. However, acertain small thickness of the coating material remaining in thecavities would not substantially influence the overall properties of thesensing device 200. In general, each cavity 206 is centered above acorresponding sensing element 204, having the same shape as the sensingelement 204, and the size of the cavity is preferably as close aspossible to the size of the sensing element 204. However, the remainingside walls between cavities must be sufficiently thick so as to maintainstructural stability. As an example, for sensing elements having a sizeof 50×50 μm, the coating layer has a thickness of approximately 30 μmand the cavities preferably have a size in the range of 30×30 to 40×40μm.

After forming the cavities 206, the coating layer 205 may be treated ina plasma cleaning process in order to improve wetting of the surface andto improve adhesion between the coating and the subsequently depositeddielectric material. The plasma cleaning may for example comprise oxygenmixed with an inert gas such as nitrogen or argon.

The dielectric material may be a mold material or an adhesive material.In the following, the manufacturing method will be discussed withreference to a dielectric mold material which is deposited bycompression molding. A dielectric material in the form of an adhesivemay be deposited in other ways, such as by dispensing a fluid adhesivematerial.

As a next step, mold granules 502 are deposited onto the coating layer205 so that the granules 502 fills the cavities, as illustrated in FIG.5 b.

Next, the mold granules 502 are heated and pressure is applied so thatthe mold material melts and the melted mold material is thereby pressedinto the cavities so that the cavities are filled with the mold material208 as illustrated in FIG. 5 c.

An optional manufacturing step is illustrated in FIG. 5d , where aprotective plate 502 is attached to the sensing device by means of anadhesive arranged between the mold material 208 and the protective plate502. In the device comprising a protective plate 502, the exteriorsurface 504 of the protective plate forms the sensing surface of thesensing device. The protective plate 502 may for example be a sapphireplate having a thickness in the range of 100-1000 μm. The protectiveplate 502 may also be the cover glass in a handheld device comprising atouch screen, and a cover glass covering the fingerprint sensing devicemay also be covering the display and touchscreen portions of thehandheld device. In principle, the protective plate may be any structurewhich acts to cover and protect the sensing device while still allowinga capacitive coupling between a finger placed on the surface of theprotective plate and the sensing elements.

FIG. 6 is a schematic illustration of a fingerprint sensing device 600according to another embodiment of the invention. The fingerprintsensing device is based on a sensing chip 202 comprising a square arrayof sensing elements 204. In many aspects, the sensing device 600 of FIG.6 is similar to the sensing device of FIG. 2a . However, the sensingdevice 600 comprises a coating layer 602 in the form of a dielectricmaterial having a plurality of trenches 604 filled with a fill material606 in the form of a mold material. Even though the followingdescription relates to providing a mold material in the trenches of thedielectric coating material, it should be noted that the fill materialequally well may be an adhesive material. The trenches 604 are alignedwith areas between the sensing elements 204. Moreover, the dielectricconstant of the mold material 606 is lower than a dielectric constant ofthe material of the coating layer 602. The coating 602, will then act asa focusing element helping to focus the electromagnetic field linesbetween a finger and the sensing element 204 towards the sensing element204 in a similar manner as discussed in relation to FIGS. 2a and 2b .Moreover, the dielectric constant of the coating can be tailored usingdielectric filler particles in the same manner as discussed above forthe mold material.

FIG. 7 is a flow chart outlining the general steps of a manufacturingmethod according to an embodiment of the invention. The manufacturingmethod will be discussed also with reference to FIGS. 8a -c.

First, in step 702, a sensing chip 202 is provided and next a coatinglayer is deposited 704 onto the sensing chip 202. The coating layertypically has a uniform thickness and is arranged to cover the entirearea of the sensing chip including the sensing elements 204. The coatinglayer can for example be a photoresist deposited by spin coating, andthe photoresist may be either a positive or a negative photoresist. Inorder to achieve a coating material having a dielectric constant higherthan the dielectric constant of the mold material, dielectric fillerparticles may be mixed with the coating material. The dielectric fillerparticles can be similar to the filler particles discussed above inrelation to the embodiment illustrated by FIGS. 2a -b.

Trenches 604 are formed 706 in the coating layer by means ofconventional photolithography and subsequent development to formtrenches having the desired shape and orientation, as exemplified inFIG. 8a . In general, trenches are aligned with areas between thesensing elements 204. The remaining coating 602 thus form squarestructures arranged on top of and aligned with the sensing elements 204.

After forming the trenches in the coating layer, the mold granules 502are provided 708 as illustrated in FIG. 8b . It should be noted that inthis embodiment, the mold material does not need to comprise fillerparticles having a dielectric constant higher than the remainder of themold material.

Next, the mold granules are compression molded 710 so that the moldmaterial fills the trenches in the coating, as illustrated in FIG. 8 c.

FIG. 9 schematically illustrates a fingerprint sensing device 900according to an embodiment of the invention. In most respects, thesensing device 900 is similar to the sensing device illustrated in FIG.2a . However, in the sensing device of FIG. 9, the cavities are smaller,meaning that the side walls 902 surrounding the cavities are thicker,and that they extend out over the sensing elements 204. In order toensure sufficient structural stability of the side walls 902, it may bedesirable to have side walls 902 which are thicker than the distancebetween adjacent elements. The thickness of the side wall may also beselected based on the technology used to form the pattern in the coatinglayer. Furthermore, the advantageous effects relating to the higherdielectric constant of the mold material in the cavities remains alsofor smaller cavities, although the effect is approximately proportionalto the size of the cavities.

The above example embodiments have been described using a photoresist asthe coating layer. However, various advantages of the present inventiveconcept are achievable using another coating material. For example, thecoating material may comprise a deposited hard mask which issubsequently patterned by for example deep reactive ion etching (DRIE).Moreover, various embodiments referring to a mold material may also beapplicable for an adhesive material.

FIG. 10a is a schematic illustration of a sensing element 204 of asensing device. Here, a rectangular cuboid structure 910 representingeither coating or mold according to the various embodiments discussedabove is arranged on the sensing element 204. In FIG. 10b , acylindrical structure 920 representing either coating or mold accordingto the various embodiments discussed above is arranged on the sensingelement 204. FIGS. 10a-b are meant to illustrate that the portionlocated above the sensing element, and which has a higher dielectricconstant that the dielectric constant of a surrounding material, may inprinciple have an arbitrary shape. The shape may for example be selectedbased on what is most desirable from a manufacturing perspective.

It should be noted that the general aspects of the invention discussedherein are not limited to the specific dimensions and sizes disclosed inthe present description. The above description merely provides anexample embodiment of the inventive concepts as defined by the claims.

Even though the invention has been described with reference to specificexemplifying embodiments thereof, many different alterations,modifications and the like will become apparent for those skilled in theart. Also, it should be noted that parts of the device and method may beomitted, interchanged or arranged in various ways, the device and methodyet being able to perform the functionality of the present invention.

Additionally, variations to the disclosed embodiments can be understoodand effected by the skilled person in practicing the claimed invention,from a study of the drawings, the disclosure, and the appended claims.In the claims, the word “comprising” does not exclude other elements orsteps, and the indefinite article “a” or “an” does not exclude aplurality. The mere fact that certain measures are recited in mutuallydifferent dependent claims does not indicate that a combination of thesemeasured cannot be used to advantage.

The invention claimed is:
 1. A fingerprint sensing device comprising: asensing chip comprising an array of sensing elements, said sensingelements being configured to be connected to readout circuitry fordetecting a capacitive coupling between each of said sensing elementsand a finger placed on a sensing surface of said sensing device; acoating material arranged in a layer on top of said array of sensingelements, said coating material comprising a plurality of cavitiesfilled with a dielectric material, wherein locations of said cavitiescorrespond to locations of said sensing elements, such that across-section area of a cavity covers at least a portion of an area of acorresponding sensing element; and wherein said dielectric materialcomprises reduced graphene oxide such that a dielectric constant of saiddielectric material is higher than a dielectric constant of said coatingmaterial, wherein the reduced graphene oxide comprises graphene oxidethat has been subjected to a reduction process.
 2. The device accordingto claim 1, wherein the dielectric material is a mold material.
 3. Thedevice according to claim 1, wherein the dielectric material is anadhesive material.
 4. The device according to claim 3, furthercomprising a protective plate attached to said fingerprint sensingdevice by means of said adhesive.
 5. The device according to claim 1,wherein a dielectric constant of said dielectric material is in therange of 5 to
 100. 6. The device according to claim 1, wherein adielectric constant of said dielectric material is in the range of 2 to5.
 7. The device according to claim 1, wherein a ratio between saiddielectric constant of said dielectric material and said dielectricconstant of said coating material is equal to or larger than 2:1.
 8. Thedevice according to claim 1, wherein said dielectric material comprisesfiller particles comprising reduced graphene oxide, the filler particleshaving a dielectric constant higher than an average dielectric constantof said mold material.
 9. The device according to claim 1, wherein saiddielectric material comprises a weight/volume percentage of reducedgraphene oxide in the range of 0.01 to 10 wt %.
 10. The fingerprintsensing device according to claim 1, further comprising openings betweenadjacent cavities so that a liquid dielectric material can flow betweenadjacent cavities.
 11. A fingerprint sensing device comprising: asensing chip comprising an array of sensing elements, said sensingelements being configured to be connected to readout circuitry fordetecting a capacitive coupling between each of said sensing elementsand a finger placed on a sensing surface of said sensing device; adielectric material arranged in a layer on top of said array of sensingelements, said dielectric material comprising reduced graphene oxide,said dielectric material further comprising a plurality of trenchesfilled with a fill material, wherein locations of said trenchescorrespond to areas between adjacent sensing elements; wherein adielectric constant of said fill material is lower than a dielectricconstant of said dielectric material, wherein the reduced graphene oxidecomprises graphene oxide that has been subjected to a reduction process.12. The device according to claim 11, further comprising a protectiveplate attached to said fingerprint sensing device by means of anadhesive.
 13. The device according to claim 11, wherein a dielectricconstant of said dielectric material is in the range of 5 to
 100. 14.The device according to claim 11, wherein a dielectric constant of saidfill material is in the range of 2 to
 5. 15. The device according toclaim 11, wherein a ratio between said dielectric constant of saiddielectric material and said dielectric constant of said fill materialis equal to or larger than 2:1.
 16. The device according to claim 11,wherein said dielectric material comprises filler particles comprisingreduced graphene oxide, the filler particles having a dielectricconstant higher than an average dielectric constant of said fillmaterial.
 17. The device according to claim 11, wherein said dielectricmaterial comprises a weight/volume percentage of reduced graphene oxidein the range of 0.01 to 10 wt %.
 18. The device according to claim 11,wherein said fill material is an adhesive or a mold material.